Question

The optical prescription of a pair of spectacle is - Right eye : - 3.5 D, Left eye : - 4.00 D
i) Name the defect of the eye
ii) Are these lenses thinner at the middle or at the edges?
iii) Which lens has a greater focal length?

Answer 1

(i) These lenses have negative power. So the person is nearsighted. Hence he has Myopia.

(ii) As the lenses have negative powers, so the focal length is negative. Hence they are concave in nature. And concave lenses are thinner in the middle.

(iii) We know,

Power of lens, $P=\frac{1}{f}$

As power and focal length are inversely proportional to each other, so the lens with lower power will have a greater focal length. right eye lens has a greater focal length.